To fix a bug where specifying multiple UTXOs that are by themselves
large enough to satisfy the output amount would lead to the rest of them
being added to fees, we need to provide the transaction author with a
constant list of UTXOs. If we didn't, the author would only consider one
input and calculate the change based on that alone. But since we'd add
all inputs to the PSBT, the rest of the amounts would go to fees.
To make it easy to show the user what change output was created (if any)
during the funding process, we return its index (or -1 if no change
output was created).
